Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Depends on how coarse the substrate it.
In gravel there is more water flow, so the slow release tablet will release their nutrients faster, and these nutrients tend to end up in the water column. I think 1.5" is not deep enough with gravel.
In a finer substrate with high cationic exchange capacity there is less water flow, so the tablets release their nutrients slower, and the substrate holds on to them better, so a lot less gets into the water column. I think 1.5" is plenty with a fine, high CEC material.
You can try just one or two tablets. Then monitor the water conditions. If too much ferts are getting into the water then you know not to use more.